Reengineering the Prosthetic-Arm Socket

To create the next generation of prosthetic arms, Dean Kamen had to reinvent the prosthetic socket

15 February 2008The prosthetic arm hasn't changed much since the second World War. It's basically a cable and a hook that opens and closes when you shrug. With only 6000 people needing new prosthetic arms in a given year in the United States, the market has not been big enough or lucrative enough to warrant expensive improvements. But Segway inventor Dean Kamen took on the task of creating a better device at the behest of the U.S. Department of Defense. His team of engineers at Deka Research & Development Corp., in Manchester, N.H., have developed a state-of-the-art functional prosthetic arm with usable fingers and sensory feedback. They were able to do so largely because of advanced low-power electronics and better batteries. But one part of the problem had little to do with motors or processing and everything to do with changing a key part of current prosthetic design: the way the prosthetic is joined to the body. Commonly known as the socket, that interface is the No. 1 complaint of arm amputees.

The goal of the Defense Department project, started in 2005, was to create an intuitively usable cutting-edge arm. But Kamen soon realized that an improved arm would be useless if it didn't feel as naturally connected to the body as a real arm. ”The problem wasn't entirely the arm,” Kamen says. ”It was how the arm attaches to the person. We spoke to a lot of people with sockets and we found 80 percent of the current kinds sit in people's closets.”

The traditional socket is modeled on the theory that the more material that is in contact with the residuum—the healed residual limb—the better the hold. ”The old paradigm was, let's have as much surface area connecting as possible,” says Deka biomedical consultant David Altobelli. Prosthetists would take a mold of the residuum, and the socket of the prosthetic was custom-fit to that mold, the idea being that the two would slide into place, connecting as seamlessly as puzzle pieces.

In theory, this works. But the mass distribution of the human body changes subtly over the course of a day. Think of wearing a pair of shoes all day. Walking a few miles in them causes the fit to change noticeably. Now imagine bare skin chafing inside those shoes. Also, over the course of a normal day, skin exudes a small amount of moisture that evaporates too quickly for the average person to notice. But inside a closed socket, even the slightest bit of sweat has nowhere to go, and with nothing to absorb it, the socket gets hot, sweaty, and slippery. That's why patients like Chuck Hildreth opt to let their prostheses collect dust in the back of the closet. ”It's frustrating,” he says. ”You can't do much with the arm because your stump is actually moving in the socket.” Hildreth, who lost both of his arms 26 years ago, now helps Deka test its Luke arm, including the socket.

He uses the socket on his left residuum, where his arm was amputated between the elbow and the shoulder. The ideal prosthetic would let the patient partially control the artificial extension with his remaining native limb. So how do you secure the arm as tightly as if it were a natural extension of the bone but avoid the slipping and chafing that come with that tight connection?

Altobelli's engineers recreated the socket from the shoulder blade out. First they decided that the shape should adjust itself to the user, but that contradicted the surface-area paradigm. So they replaced the traditional single continuous shape with separate fingerlike claws. They called their design the active socket.

The torso attachment mechanism looks a bit like a harness: a solid carbon-fiber breastplate is wrapped around the body and strapped to a counterplate on the other side of the body to secure it. To prevent chafing, a rubbery green silicone sheet under the carbon-fiber plates is riddled with air pockets that inflate slightly to keep the wearer comfortable.

Attached to the harness at the shoulder is a basket-shaped carbon-fiber casing made of five widely spaced prongs. The design is open, the space between the prongs allowing air to circulate freely. Hildreth slips his residuum into this casing. Though firmly attached to the body's harness, the casing allows the wearer to move his stump freely inside. Five thin urethane film bladders Velcroed inside the prongs inflate around the residuum to the point that the soft tissue is moved out of the way and the casing is gripped firmly onto the bone. The end result looks like a big carbon-fiber claw gripping Hildreth's stump. ”The mechanism is incredibly strong,” says Altobelli. ”We once used it to pick Chuck up off the ground.”

”When he wants to move the bone, it doesn't have to delay through squishy soft tissue,” says Altobelli. At the bottom of the socket, a threaded coupling screws into the Deka arm.

The arm retains the same amount of control regardless of how much weight Hildreth puts on it. The bladders are controlled by a load-sensing microprocessor that adjusts on the fly to changes in pressure. When a patient picks up something heavy, like a suitcase, the bladders on the claw's individual fingers inflate, tightening their hold on the bone commensurate with the load being lifted. Once the patient puts down the load, the bladders deflate slightly for comfort.

Now Hildreth can lift his prosthesis over his head, a feat previously impossible with such a short residuum. ”I have no sweating,” he says, swinging his arm up. ”I don't lose friction, so I can control the arm.”

The new socket revolutionizes the Deka arm, which Kamen hopes to bring to clinical trials this year, and which should be available within two years. But it could also be used separately with standard, currently available prosthetics to improve patients' control, says Altobelli. In fact, the socket could work for prosthetic legs. Even the most advanced artificial leg on the market, the microprocessor-enhanced C-Leg, has the same kind of silicone socket that gives arm patients so many problems. The Deka socket could end up being the most game-changing element of an already game-changing arm.